This invention relates to an image forming apparatus, and particularly to an arrangement for fixing a recording material such as toner to a recording medium.
In an image forming apparatus, a recording material such as toner, ink or the like is used to form an image on a recording medium such as a paper. The recording medium with the image formed thereon is nipped by heated rollers so that the recording material is fixed to the recording medium. There is a type of image forming apparatus that has a duplex printing function, i.e., a function to form images on both sides of the recording medium.
In the image forming apparatus of this type, a plurality of recording media with no image formed thereon are stacked in a feeding tray. The recording medium is fed from the feeding tray to an image forming unit, and an image is transferred to a first side of the recording medium. The recording medium is then fed to a fixing unit, and the image is fixed to the first side of the recording medium by means of the rollers of the fixing unit. After the image is fixed to the first side of the recording medium, the recording medium is turned upside down by a turning mechanism, and again fed to the image forming unit by a duplex feeding mechanism. In the image forming unit, an image is formed on a second side of the recording medium.
After the image is formed on the second side of the recording medium, the image is fixed to the second side of the recording medium as in the case of the first side of the recording medium. Then, the recording medium is ejected through an ejection path, and a duplex printing is completed. As described above, in the duplex printing, there is a time interval between the fixing operations of the first side and the second side of the recording medium. Such an image forming apparatus is disclosed in, for example, Japanese Kokai Publication No. 2000-338817 (in particular, page 4 and FIG. 1).
However, because of the above described time interval between two fixing operations, upper and lower rollers of the fixing unit directly contact each other and rotate with each other for a relatively long time. Therefore, heat may transfer, for example, from the upper roller to the lower roller, and therefore the temperature of the lower roller becomes higher than a target temperature even when a heater built in the lower roller is turned off. During the fixing operation of the second side, the first side contacts the lower roller, and therefore the image that has already been fixed to the first side may be molten because of the high temperature of the lower roller. Therefore, the fixed image may become uneven and a hot offset may occur. The hot offset is a phenomenon that a part of the recording material adheres to the excessively heated roller (i.e., the lower roller) and separates from the recording medium.
Moreover, in a simplex printing in which the image is formed on only one side of the recording medium, if there is a relatively long time interval between fixing operations of the preceding recording medium and the subsequent recording medium, the lower roller may be excessively heated. In such a case, the brightness of the image may be unstable, and the image quality may be degraded.